Resonant and semi-resonant DC-DC converters, including isolated and non-isolated topologies, are used in a variety of applications including telecommunications, consumer electronics, computer power supplies, etc. The usage of such converters is gaining popularity because of their zero-voltage (current) switching characteristic and their ability to utilize parasitic elements inherent in an electronic circuit. Among numerous topologies, the semi-resonant converter with transformer/tapped-inductor is an attractive topology for providing high voltage conversion ratios without using isolation. Such converters provide advantages including lower cost and higher efficiency as compared to other solutions.
A semi-resonant converter typically includes high-side and low-side switches that transfer power from an input source to a transformer/tapped-inductor that supplies output power to a load. The transformer/tapped-inductor is also connected to a second low-side switch device, which is termed a synchronous rectification (SR) switch herein. Due to the “on resistance” of the SR switch, some power will be lost in this switch when current flows through it. For example, a metal-oxide semiconductor field-effect transistor (MOSFET), which is a common switch device used in such applications, is characterized by a drain-to-source resistance Rdson in its “on” or conducting state. The power losses can become significant when high currents are flowing through the SR switch. Given that the semi-resonant converter with a transformer/tapped-inductor offers significant advantages for applications requiring a high input-to-output voltage ratio (i.e., a large step-down voltage), such converters are commonly used to convert high-voltage/low-current power into low-voltage/high-current power. The high output current for such usage translates into high current through the SR switch which, in turn, can lead to significant power loss.
Prior techniques for addressing such power loss in power converters have focussed on reducing the effective series resistance (ESR) of the SR switch. One such technique uses larger switches, which typically have lower “on” resistance. Another technique, which may be used in conjunction with the first technique, is to connect a plurality of SR switches in parallel and to control this plurality of SR switches using a common control signal. The effective “on” resistance of the parallel SR switches is thus reduced. Such solutions work well when the output current of a semi-resonant power converter is high, such as when it is under heavy load. However, such solutions are suboptimal for situations in which the output current is relatively low, e.g., when a power converter is only lightly loaded. For this case, the losses associated with the control signal, e.g., gate drive losses or gate charge losses, become significant relative to the conduction loss of the SR switch(es). A larger switch device typically has higher losses associated with its control, and increasing the number of power switches proportionally increases such losses. Considering, for example, the case where the SR switch(es) is/are one or more MOSFETs, the gate charge losses (characterized using the gate resistance Rg of the MOSFET) and gate drive losses become larger than and dominate the conduction losses (characterized by the “on” resistance Rdson) when the power converter is lightly loaded, i.e., when the power converter is providing relatively low output current.
Accordingly, there is a need for an improved technique that reduces the power loss for SR switches in a power converter when the power converter is supplying high as well as low output currents. Such techniques should apply both to single and multi-phase power converters.